1. Field of the Invention
The present invention relates in general to a laminated piezoelectric/electrostrictive driver which provides a displacement due to a strain caused by a longitudinal mode effect of a piezoelectric or electrostrictive ceramic material, and also relates to a printing actuator using such a piezoelectric or electrostrictive driver for driving a printing element such as a print wire for a dot-matrix printing, for example.
2. Discussion of the Prior Art
A lamination type piezoelectric or electrostrictive element utilizing the longitudinal mode of the reverse piezoelectric effect or the electrostrictive effect. This laminar longitudinal-effect type piezoelectric or electrostrictive element consists of a plurality of piezoelectric ceramic films of lead zirconate titanate (PZT), for example, and a plurality of electrode films, which are alternately superposed on each other, in the direction in which the element is displaced (contracted and expanded) upon application of a voltage to the electrode films. The piezoelectric or electrostrictive element which is thus displaced is used as drivers for a dot-matrix impact print head, as an actuator for a VTR head, and an oscillator for a ultrasonic motor or other devices. In particular, there has been a growing attention to an actuator laminated in a process used for a laminar type ceramic capacitor, for its relatively high production efficiency and operating reliability.
An example of a piezoelectric printing actuator is generally shown at 100 in FIG. 5. This printing actuator 100 employs a generally elongate laminar longitudinal-effect type piezoelectric driver 101 whose linear displacement upon application of a voltage is utilized to drive a print element in the form of a print wire 11. The displacement is produced due to the longitudinal mode of the reverse piezoelectric effect or the electrostrictive effect, which is known in the art. The "longitudinal mode effect" means the reverse piezoelectric effect or the electrostrictive effect parallel to the direction of the electric field produced by the voltage applied to the driver, and is distinguished from the "transverse mode effect" perpendicular to the direction of the electric field. In the example of FIG. 5, the displacement occurs in the longitudinal direction of the generally elongate driver 101 in which piezoelectric ceramic films 42 are superposed, as shown in FIG. 6.
As shown in FIG. 5, the printing actuator 100 has a support holder having a main frame 2 provided with a base support block 3 at one end thereof. The piezoelectric driver 101 is supported by and accommodated in the support holder such that the driver 101 rests on the support block 3 at one of its longitudinal ends. A movable member 5 is fixed to the other end of the driver 101. The support holder also supports a link mechanism 16 disposed so as to guide the movable member 5 in the longitudinal direction of the driver 101 when the driver is displaced in the longitudinal direction. The movable member 5 is linked with a mechanism for amplifying the linear movement of the driver 101 into a larger longitudinal movement of the print wire 11. This converting mechanism includes a pair of sheet springs 6, 7, a tiltable member 8, and an arm 10. The sheet spring 6 is fixed at one end thereof to the main frame 2, while the sheet spring 7 is fixed at one end thereof to the movable member 5. The other ends of the sheet springs 6, 7 are fixed to the tiltable member 8, which is secured to one end of the arm 10. The print wire 11 is fixed at one end thereof to the other end of the arm 10. The linear displacement of the driver 101 produced upon application of a voltage thereto causes the sheet spring 7 to be displaced, thereby causing the tiltable member to be tilted. The tilting movement of the tiltable member 8 are transmitted to the print wire 11 through the arm 10, whereby the print wire 11 effects a printing movement toward a recording medium.
An example of the known laminar piezoelectric driver 101 is illustrated in FIG. 6. The driver 101 has 180 piezoelectric ceramic films 42 and 181 electrode films 41 which are alternately superposed on each other. Each piezoelectric ceramic film has a thickness of 98 .mu.m, and each electrode film 41 has a thickness of 2 .mu.m, so that the laminar piezoelectric driver 101 has a total thickness of about 18 mm. The piezoelectric ceramic film 42 has a piezoelectric constant d.sub.33 of 6.35.times.10.sup.-10 m/V, and an electrostrictive constant of M.sub.33 of 1.32.times.10.sup.-16 m/V.sup.2. For the piezoelectric driver 101 to provide a linear displacement of 15 .mu.m necessary to give a required movement to the print wire 11, 107V should be applied to the electrode films 41 to polarize the piezoelectric films 42 as indicated by arrows 45 in FIG. 6. Namely, the laminar piezoelectric driver 101 undergoes a linear displacement in its longitudinal direction (in the direction of lamination of the films 41, 42), due to the longitudinal mode of the reverse piezoelectric effect or electrostrictive effect parallel to the direction of the polarization.
The piezoelectric driver 101 has a property that the amount of residual strain remaining after the polarization of the piezoelectric ceramic films 42 is reduced as the temperature of the films 42 is elevated during use of the driver 101. Due to this phenomenon, the driver 101 has a very low or negative coefficient of linear thermal expansion (e.g., -3.8 ppm/.degree.C.) at the operating temperature, unlike the other components of the printing actuator 100. Therefore, there exists a difference in the linear thermal expansion coefficient between the driver 101 and the main frame 2, which should be compensated for to assure reliable operation of the printing actuator 100. To this end, the main frame 2 is made of a metallic material having a low coefficient of linear thermal expansion, for example, an Invar alloy whose linear thermal expansion coefficient is +1.2 ppm/.degree.C., and rigid temperature compensating members 12, 13 having a high positive linear thermal expansion coefficient are used in contact with the lower end of the driver 101. For instance, the temperature compensating members 12, 13 are aluminum blocks having a total thickness of 4 mm and a linear thermal expansion coefficient of +23.9 ppm/.degree.C.
However, the material having a low linear thermal expansion coefficient for the frame 2 is considerably expensive, leading to an increased cost of the printing actuator 101. Further, it is cumbersome and time-consuming to position the temperature compensating members 12, 13 relative to the support block 3 and the piezoelectric driver 101. Thus, the use of the temperature compensating members 12, 13 pushes up the cost of manufacture. Further, there is a risk that the temperature compensating members 12, 13 are separated from the piezoelectric driver 101 during use, due to low bonding strength therebetween.
Where the piezoelectric driver 101 does not have a sufficiently high thermal conductivity, there may arise a temperature difference between the driver 101 and the temperature compensating members 12, 13. This temperature difference reduces the function of the temperature compensating members, and may cause an undesirable phenomenon that the operating end of the print wire 11 is advanced toward the recording medium as the operating temperature of the driver 101 increases. This results in the the tip of the print wire 11 contacting an ink ribbon or the recording medium while the print wire 11 is in the non-operated position.
The known piezoelectric driver 101 also suffers from a relatively high voltage (107V in the example of FIG. 6) required to obtain the desired printing movement of the print wire 11. One method of lowering the required voltage is to reduce the thickness of each piezoelectric ceramic film 42 and increase the number of the piezoelectric ceramic films. However, this solution suffers from another problem in sintering the driver 101. That is, there exists a difference in the sintering temperature of the driver 101, between the piezoelectric films 42 relatively near the longitudinal ends of the driver 101 and the films 42 in the middle of the lamination. This tends to lower the uniformity in the physical properties and the operating reliability of the driver 101.